1. Field of the Invention
This invention relates to light absorbing, low reflectance coatings, primarily but not limited for use in light absorbing baffle coatings, solar energy collectors and photothermal devices in general.
2. Brief Description of the Prior Art
Infrared detectors and optical elements frequently require an opaque low reflecting (light absorbing) coating in several locations for proper operation. The coating should be as thin as possible but certainly less than 5 micrometers thick, transmit less than 0.1% and reflect less than 0.5% at wavelengths near 10 micrometers and preferably in the entire 8 to 14 micrometer range.
Opacity in the prior art has generally been defined to require a relatively thick layer (100 to 1000 atoms thick) of a metal having thereon a quarter wave length thick (in the wavelength region of interest) layer of a transparent dielectric and a thin semi-transparent layer thereover forming a resonating absorbing structure. The light paths established which are reflected from either the top or bottom of the quarter wave length thick layer destructively interfere with each other because the light reflected from the top of this layer is 180.degree. out of phase with the light reflected from the bottom of this layer. This causes a cancellation or near cancellation of the reflected light to produce low reflectance. The addition of a further layer of dielectric material further reduces reflectance, improves the durability of the coating and increases the width of the low reflecting band. The thickness of quarter wave layer and the semi-transparent metal layer are altered to create a structure with an optical impedance which can be optimally anti-reflected by the further layer of dielectric material. All layers are preferably thin slabs of homogeneous material having densities approaching that of bulk material with mathematically sharp boundaries. The above described design for producing low reflectance in the visible spectrum was first published by Haas, Turner and Schroeder, Journal of the Optical Society of America, Volume 46, page 31 (1956).
The above described design was utilized using materials other than those described in the Haas et al. publication to provide low reflectance in the 0.5 to 3.0 micrometer range in about 1976. A design of the above general structure was described to the Air Force in 1979 using layers of titanium and aluminum oxide to produce low reflectance in the infrared region. Since 1985, yet another version of the above described design has been used to produce low reflectance at wavelengths near 10 .mu.m with layers of titanium, germanium and zinc sulfide. A lift-off method is used for feature delineation.
The above described design for use near 10 .mu.m relies upon the thin semitransparent metal layer having a thickness of about 15 nanometers. If the physical thickness varies by .+-.1 nanometer (about 4 atoms), the thickness of the fourth layer must usually be altered to bring the measured spectrum close to that intended. The region of very low reflectance is only about 1 .mu.m wide. The reflectance increases as the angle of incidence changes from 0.degree. (perpendicular to the surface) to any other angle, such as, for example, 45.degree.. The above described design requires that the optical properties of the thin layer of reactive metal, titanium, be accurately known and reproduced exactly on a run-to-run basis to produce high yield. Slight changes in the deposition technique or residual atmosphere in the vacuum chamber alters the properties of the thin semi-transparent metal layer. This propensity, when combined with the thickness sensitivity, explains the relatively low probability of achieving the theoretically predicted response and accounts for "settling" for higher reflectance on a daily basis.
Spongy metal film structures are described by D. Slocum who utilized oblique evaporation of silver to make needle-like polarizing layers. Metal "soot" films have been produced by evaporating at higher pressure in non-reactive gases with substrates very close to the source. These films are easily rubbed or even blown off the substrates. W. Lang has described in Journal of Vacuum Science Technology, Nov/Dec 1990, evaporation of gold in nitrogen and aluminum in argon. Lang adds 20% oxygen to the argon while evaporating aluminum in a total pressure of 1.5 Tort. The density of these aluminum "soot films" is 250 .mu.g/cc, or about 1/10,000 the density of bulk aluminum.